The following invention relates to a bearing guided ferrofluid seals and unique seal carriers for use in electric spindle motors having bearing assemblies with an outer annular ring surface.
Conventional electric spindle motors of the type used in disk drives conventionally use ball bearing assemblies to facilitate movement between a rotary member and a stationary member. As shown in FIGS. 2 and 3, bearing assemblies 18 generally include metallic or ceramic ball bearings 24 that are positioned between an inner bearing race or ring 20 and an outer bearing race or ring 22. Bearing assemblies 18 may be either inner or outer rotators depending on whether the shaft 28 (which is substantially adjacent the inner bearing ring 20) is stationary or whether a combination of the shaft 28 and hub 32 rotate together. Inner rotators have an inner bearing ring 20 that rotates with the shaft 28. Outer rotators have an outer bearing ring 22 that rotates with the hub 32. The ball bearings 24 are preferably evenly spaced within the inner and outer bearing rings 20, 22. The hub 32 has an inner surface 30 that may have a single circumference (FIGS. 2 and 3) or two circumferences 34, 36 (FIGS. 4 and 5).
Arguably, ferrofluid seals 40 are used on the majority of spindle motors produced today for hard disk drives. Further, ferrofluid seals 40 are now being used in other areas of the disk drive such as in the head-stack bearing cartridge assemblies, in other types of motors, and in the pivoting sections of machines that are used in high cleanliness environments. Accordingly, there is a great demand for effective ferrofluid seals 40.
Ferrofluid seals 40 are commonly used to provide a hermetic seal against gas and other contaminates in applications where a seal is needed between a shaft and its surroundings. In other words, ferrofluid seals 40 are capable of withstanding relative rotation between a shaft and its surroundings. Ferrofluid seals 40 may also be used in single seal motors in which there is a ferrofluid seal 40 at one end and a labyrinth seal or some alternative seal is at the other end.
Ferrofluid seals 40 are generally constructed of two O-shaped pole pieces 42a, 42b sandwiching an O-shaped magnet 44. The Ferrofluid 46 is positioned between the seal inner diameter 48a of the magnet sandwich and the outer diameter of a shaft 28. Magnetic flux holds the ferrofluid 46 in place. In other words, a ferrofluid seal 40 is an apparatus that includes magnetic fluid holding means for storing a magnetically permeable fluid between an inner element and an outer element, which are relatively rotated. The magnetic fluid holding means has a storage section for storing a part of the magnetic fluid that extends out of the magnetic fluid means. One exemplary ferrofluid seal apparatus is disclosed in U.S. Pat. No. 5,238,254 to Takii et al. Ferro Technologies, Inc., of Pittsburgh, Pa. produces other conventional ferrofluid seals 40. Ferro Technologies, Inc. produces several embodiments of ferrofluid seals 40 including, but not limited to, a Z-seal (FIG. 2), an OZ-seal (FIG. 3), a P-seal (not shown), and a U-seal (not shown).
Unfortunately, almost all ferrofluid seals are not perfect. The seal inner diameter 48a is not always a perfect circle and the seal outer diameter 48b is not always a perfect circle. Further, the center point of the seal inner diameter 48a and the center point of the seal outer diameter 48b are not always aligned. When the center points are not aligned then the seal inner diameter 48a and the seal outer diameter 48b are not concentric. A lack of concentricity between the seal inner diameter 48a and the seal outer diameter 48b can be referred to as runout.
The ferrofluid seals 40 are effective, but they are delicate and prone to splash and burst problems when they are installed incorrectly or contaminated due to particles or serious out-gassing. The likelihood of splash and burst problems is increased by the introduction of eccentricity between the ferrofluid seal inner diameter 48a (also called the seal operating face) and the shaft outer diameter (the surface against which the ferrofluid 46 runs). A splash or a burst is catastrophic because it ejects ferrofluid 46 into a clean environment. Further, a splash or burst may cause a complete seal failure that will then allow particles or out-gassing to pass into the same clean environment.
Ferrofluid seals 40 are generally held in or part of known traditional seal carriers 50, 52 such as those shown in FIGS. 2 and 3. FIG. 2 shows a ferrofluid seal 40 held in one example of a conventional seal carrier 50. Conceptually this conventional seal carrier 50 would be substantially washer shaped with a central annular cutout or clearance into which the ferrofluid seal 40 would fit. FIG. 3 shows an alternative example of a conventional seal carrier 52 that is an extended pole piece 42b and, therefore, is an integral part of the ferrofluid seal 40. Specifically, the pole piece 42b of the alternative carrier 52 acts as the carrier itself and drops into the bearing bore (or hub inner surface 30) or bore that is, under ideal circumstances, concentric with the axis of rotation.
Both examples of conventional carriers 50, 52 are difficult to accurately place in a motor. Further, because the ferrofluid seals are typically placed into conventional carriers 50, 52 using a clearance or slip-fit, the ferrofluid seal can fit anywhere within the clearance and, most likely will be off-center. Accordingly, the seal carriers"" additional clearances contribute to eccentricity and runout (where the seal inner diameter 48a is not concentric with the outer diameter of the carrier 50) problems.
Using the bearing for positioning is not unknown. For example, U.S. Pat. No. 5,876,126, to Marshall et al. (the xe2x80x9cMarshall Patentxe2x80x9d), which is assigned to the same assignee as the present invention and incorporated herein by reference, is directed to a motor incorporating a bearing guided labyrinth system. The motor includes a shaft, a hub, a bearing assembly, and a bearing guided labyrinth. The bearing assembly includes an inner bearing ring and an outer bearing ring separated by a plurality of ball bearings. The inner bearing ring has an inner annular ring surface and the outer bearing ring has an outer annular ring surface. The bearing guided labyrinth has an inner prong and an outer prong. The inner prong is at least partially positionable between the inner ring surface and the shaft. The outer prong is at least partially positionable between the outer ring surface and the hub. The outer prong grips the bearing assembly and specifically the outer annular ring surface of the outer bearing ring. Because the purpose of the bearing guided labyrinth system is to reduce particle emission, using the bearing to position the bearing guided labyrinth system, which allows for a tighter fit, is quite effective.
U.S. Pat. No. 5,227,686 to Ogawa includes a spindle motor embodiment in which a lower magnetic fluid sealing means is held in a holder that appears to be guided by an annular member. The lower sealing means is secured to the inner surface of the holder that is then attached to the annular member. The holder includes an annular leg that is positioned within an annular groove of the annular member. The annular groove would have to be a precision surface because it holds the holder and the lower sealing means. Accordingly, the annular groove would be difficult and expensive to produce in the annular member. The purpose of the lower sealing means is to seal the gap between the shaft and the holder and, most likely, to reduce particle emission.
The present invention is directed to bearing guided ferrofluid seals and seal carriers that substantially reduce a motor""s ferrofluid seals system""s operating eccentricity and runout without significantly reducing the strength of the joint.
The present invention is directed to a bearing guided ferrofluid seal and seal carrier system that may be used in a motor or other type of rotating device that has a bearing assembly with an outer annular ring surface. The seal carrier of the present invention has a carrier ferrofluid seal surface and a carrier bearing surface. The carrier bearing surface includes a first carrier bearing surface and a second carrier bearing surface. A ferrofluid seal may be attached to the carrier ferrofluid seal surface. The second carrier bearing surface may be attached to the outer annular ring surface. In one preferred embodiment the first carrier bearing surface may be attached to the upper annular ring surface. In a separate preferred embodiment the ferrofluid seal may be attached to the carrier ferrofluid seal surface so that the seal inner diameter of the ferrofluid seal is fixtured to the second carrier bearing surface to reduce eccentricity between the two diameters.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.